This could be any number of reasons and without knowing anything about your environment it is very difficult to comment. I would certianly start by using a performance monitoring tool to get the average RPC latancy reading. Also try other tools such as EXBPA. Is this issue just with one mailbox? If so is there something on the client machine that is submitting high RPC requests to Exchange? Some add in to Outlook maybe, or even a mobile device...? Really need more information.